1. Field of the Invention
The present invention relates to pulsed electromagnetic sensors, and more particularly to fluid and material level sensors using pulse width modulated (PWM) time-domain reflectometry (TDR). These sensors can be used for (but are not limited to) determining or controlling the material level in a tank, vat, irrigation ditch, silo, pile, or conveyor.
2. Description of Related Art
Ever since time domain reflectometry was first used in liquid level gages, as seen in U.S. Pat. No. 3,703,829, Liquid Quantity Gaging System, to Dougherty, TDR-based level sensors have grown in popularity. The TDR technique involves connecting a time-domain reflectometer to a transmission line (or probe) immersed in a liquid. The TDR measures the round-trip delay of a pulse propagating from the TDR to the liquid surface and reflecting back to the TDR. The propagation delay is independent of air temperature, pressure and humidity, and the precise location of the pulse reflection depends only on the location of liquid surface and is independent of the liquid""s dielectric constant.
The probe can be a hollow coaxial structure (except for the normal center conductor) which fills with the liquid so a TDR reflection occurs at the liquid surface within. The probe can be simplified to a single-wire transmission line, as disclosed in U.S. Pat. No. 3,995,212, xe2x80x9cApparatus and Method for Sensing a Liquid with a Single Wire Transmission Linexe2x80x9d to Ross.
The disadvantages to a single-wire transmission line are (1) high line impedance, making reflection-free matching to common 50-ohm interconnect cable impossible, (2) low reflection amplitude, making ringing and other aberrations more dominant, (3) sensitivity to nearby objects such as tank walls, (4) susceptibility to RF interference (no shielding), and (5) inability to form a PWM signal directly on the line, thereby requiring more complicated and less accurate pulse detectors.
Coaxial probes, while more complicated than single wire probes, have their advantages: (1) they produce strong reflection amplitudes, which is an advantage with low dielectric constant materials, (2) they exhibit stilling action, wherein sloshing is less pronounced inside the coaxial probe so steadier measurements can be obtained, and (3) they have natural shielding against the effects of nearby walls and radio frequency interference. The primary drawbacks to coaxial probes are (1) potential to become clogged, (2) greater mechanical complexity relative to a simple wire, (3) difficulty in segmenting or coiling long lengths for shipping, and (4) difficulty in cutting a long length to a custom length at the customer""s location to suit specific tank dimensions. In spite of these limitations, the advantages to the coaxial probe outweigh the disadvantages in many applications, particularly where the probe length is on the order of a meter or less.
The simplicity of the TDR probe and the constancy of the speed of light (at which the TDR pulses travel), make TDR an ideal level sensing technology. Unfortunately, TDR can be expensive and of limited accuracy (0.1-1%). In order to simplify the TDR apparatus while improving its accuracy, TDR pulse processing and timing must be advanced beyond the present state.
The concept behind the present invention is (1) to set up the TDR pulses on the transmission line to directly form a PWM pulse, perhaps on a millivolt scale, (2) to detect the transmission-line PWM pulse and convert it to a logic-level PWM pulse, and (3) to process the logic-level PWM pulse by using it to gate a range counter to obtain a level indication, or by averaging it into a voltage or current proportional to level. In short, the present invention passes a transmission-line PWM pulse directlyxe2x80x94and accuratelyxe2x80x94to the output as a digital PWM pulse without altering the PWM pulse duty cycle. This xe2x80x9cdirect-PWMxe2x80x9d process can be implemented in a realtime mode, where the digital output PWM pulse has a realtime scale, or in an expanded-time mode, where the digital output PWM pulse has been expanded in time by a large factor to facilitate high accuracy detection and range counting.
In the realtime mode, the transmission-line PWM signal is threshold detected to form a realtime digital PWM signal having an amplitude set by the power supply voltage Vs, a PWM width set by the transmission-line PWM signal, and a period Tclk set by the TDR clock. The realtime digital PWM signal can be averaged with a simple low pass filter to obtain a voltage Vave that is linearly proportional to the height of the liquid. Vave is a ratiometric voltage, Vave/Vs=PWM/Tclk. Vs can be set to high precision, or preferably, it can be used as a reference to a ratiometric A/D converter so its amplitude does not matter. The clock period, Tclk can be set to high accuracy with a quartz crystal oscillator. Thus, the realtime mode can be precise, in principle. In practice, propagation delay variations and other shifts in the realtime threshold detector limit accuracy. Yet, the extreme simplicity of the realtime PWM TDR apparatus is hard to ignore. It is the best mode for low cost and moderate accuracy (a few percent) tank gages or for fixed fill/overfill switches.
In the expanded-time mode, the transmission-line PWM signal is sampled to produce an expanded-time (ET) PWM signal that is a replica of the transmission-line PWM signal, except on a vastly expanded time scale. The expanded-time technique employs a stroboscopic sampling effect to convert nanosecond PWM signals to millisecond PWM signals for more accurate processing.
The expanded-time mode can be implemented in either of two ways: (1) using a single clock signal that is split into a transmit clock signal and a sampling (or receive) clock signal with a swept phase-difference between them, wherein the receive clock signal operates a sampler to produce a time-expanded PWM signal; and (2) using separate transmit and sampling (or receive) clock oscillators with a floating, i.e., unregulated, offset frequency between them to provide a steady phase slip.
The scale-factor accuracy of the two-oscillator technique is set by the accuracy of a crystal-controlled transmit clock, or xcx9c0.003% without calibration during manufacture. With either expansion process, the sampled PWM signal is threshold-detected to produce an expanded-time digital PWM signal.
The present invention can be used as an electronic dipstick for innumerable applications in material level sensing. In combination with a valve, it can be used to control or automatically regulate the level in a tank, for example. As a linear displacement transducer, where the coaxial probe is configured as a piston and cylinder assembly, vehicle height can be sensed or pneumatic/hydraulic cylinder displacement can be measured for safety or automatic control.